Rewrite generation of _get_item_

[jpfeuffer]

First of all we should support get_item with e.g. strings for dict like objects.

But, even now for just size types it is not really correct https://github.com/OpenMS/autowrap/blob/570a6f4788e6520d022959a4f40e3299ee1e7c12/autowrap/CodeGenerator.py#L1670

It should first check for invalid indexes and only then convert to a fitting datatype. Currently we get signedness warnings because it is still a signed long type even after checking positivity.